Circuit arrangement for power control

ABSTRACT

The power drawn by a load (R L ) from an A.C. network is controlled by an operational amplifier (OP1) and a power transistor (TR2). The operational amplifier makes the transistor leading when the voltage of its inverting input which is derived from said network, is lower than a reference voltage supplied to its non-inverting input. The pulses through the load take a form corresponding to the first and last parts of a sinusoidal half-wave. Their time lengths are shortened according as the voltage of the feeding network increases, and vice versa. FIG. 1.

The invention relates to a circuit arrangement for maintaining the powerdrawn by a load from an alternating current source at a value to a highdegree independent of the voltage of said source.

Circuit arrangements for said purpose are known, in which a D.C. voltagederived from an A.C. network by rectification is periodicallyinterrupted so as to form square-shaped pulses, the length of which isshortened according as their amplitude increases, the pulse length beingcontrolled by an operational amplifier comparing the fluctuating voltageof the network with a fixed reference voltage.

On account of the complicated and non-linear relationship between theload output and said voltage, it is possible to maintain the outputfairly constant merely within a comparatively narrow range of voltages.

The object of the invention is to create a circuit arrangement which,with the use of a small number of components, provides such acompensation of voltage variations of the current source that the powerdrawn by the load is maintained constant with very small and acceptabledeviations, when the voltage of the power source varies between valuesof the ratio 1:2 or within a still greater range.

The invention is based upon the experience that said power control iseasier to realize if the load is fed with pulses formed by those partsof the sine curve of the voltage of the power source that lie next tozero voltage, whereby due consideration can be taken to the fact thatwhen varying the voltage of the power source the energy contents of thepulses vary with the first power of their lengths and with the secondpower of their amplitudes.

The characterizing features of the invention are set forth in thefollowing claims, and the details of an exemplary embodiment of thecircuit are described herein below with reference to the accompanyingdrawing, in which

FIG. 1 is a circuit diagram and

FIG. 2 shows curve shapes of feeding voltages and output pulses.

The lines AC IN of the circuit are connected to a power source which isassumed to be a distribution network of 110 V and 50 Hz. By means of thediodes D1, D2, D3 and D4 the power supply is rectified, so that betweenthe lines a and c (which may be grounded) there is a potentialdifference of the curve shape shown in FIG. 2 D, and consisting ofconsecutive sinusoidal half-waves of the same positive polarity. A zenerdiode Z1 shunted by a condenser C1 is in series with a resistor R1connected between the lines a and c and holds the point or line b at aconstant, comparative low D.C. voltage (e.g. of the order of 10 V). Anoperational amplifier OP1 (e.g. of the type RCA 3140) is fed from theline b.

A resistor R3 (e.g. 100 kohm) in series with a resistor R4 (e.g. 47kohm) is connected between the lines b and c. At the point or line d avoltage of constant value of the order of 3 V is maintained and issupplied to the non-inverting input (+) of the operational amplifier OP1as a reference voltage.

The inverting input of OP1 is connected to the intermediate point e upona voltage divider consisting of a series connection of a fixed resistorR3 (330 kohm), a fixed resistor R6 (18 kohm) and an adjustable resistorR7 (50 kohm). In the point e there appears a voltage of the same fullyrectified sinusoidal shape (FIG. 2D) as in line a but of a considerablyreduced magnitude (about a tenth thereof).

The operational amplifier OP1 is connected via a resistor R8 (47 kohm)to a power transistor TR2 (e.g. of the type Siemens BUZ 45) and triggersthe same. The output terminals of said transistor are in series with theresistor R9 (0.1 ohm) and the load R_(L) connected between the lines aand c. The load is resistive and may consist of a halogen incandescentlamp rated 20 W and 24 v. The transistor TR2 shuts the load R_(L) on andoff momentarily at intervals determined by the operational amplifier.

The circuit so far described operates in the following manner: As longas the voltage of point e supplied to the inverting input of theoperational amplifier is less than the constant voltage supplied viapoint d as a reference voltage to the non-inverting input of theoperational amplifier, the output thereof supplies a voltage to thetransistor TR2 making the same leading. When the voltage upon theinverting input of OP1 grows to be the greater and as long as it remainsgreater than the voltage upon the non-inverting input of the operationalamplifier, the transistor TR2 is choked and no current flows through theload R_(L). Therefore, the load will be fed by mainly triangular pulsesof the shape shown in FIG. 2 C and consisting of the starting and endingportions of each sinusoidal half-wave of the voltage of the rectifiedpowersupply. If said voltage rises, as when connecting the lines AC Into a distribution network of 220 V, the voltage at e and upon theinverting input of the operational amplifier will rise from zero valuequicker than before to the same value as the voltage upon thenon-inverting input, so that the operational amplifier reaches itscut-off point earlier and the lengths of the pulses are shortened. Thepulses will take the shape of FIG. 2 A, being not only shorter but alsohigher than in FIG. 2 C, due to the raised voltage of the power supply(FIG. 2 B). Thus the length of the pulses is varied in order tocounteract the variations of the load output due to changing voltages ofthe power source. The circuit so far described will do as long as thevoltage of the power source varies within narrow limits, but e.g. for adoubling thereof the time length of the pulses will become too short andneeds correction. According to a secondary feature of the invention sucha correction is provided which consists in applying upon thenon-inverting input of the operational amplifier a superposed,comparatively low, smoothed portion of the voltage of the fullyrectified power supply. This is accomplished by the added components R2and C2. R2 is a resistor (e.g. 2.2 Mohm) connected between the lines aand d, and C2 is a condensor (0,47 μF) connected across the resistor R4.The fluctuating current flowing through R2 and R4 is smoothed by thecondensor C2 and raises the potential difference across R4, C2. Thus thevoltage supplied by the line d to the non-inverting input of theoperational amplifier is composed of the above-mentioned constantcomponent determined by the zener diode Z1 and a superposed variablecomponent determined by the A.C. voltage of the feeding network. Saidcomponent adjusts the reference voltage level on OP1 and the timelengths of the cut-off parts of the sine curve admitted through the loadare changed in such a manner that the energy contents of the pulses varyonly very slightly with the changed A.C. voltage. For instance, whensaid voltage is doubled, the pulse lengths are shortened to about 60-65percent.

D5, D6 and D7 are clamping diodes. TR1 is a transistor (e.g. of the typeBC 107) serving for the protection of the lamp R₁ against overload dueto possible unforeseen incidents. Said transistor is not essential tothe invention and may be omitted.

Primarily the invention aims at maintaining the power of the load at afixed value, e.g. a value at which for instance an incandescent lampgives off full light intensity. However, by adjusting the resistor R7 itis also possible to maintain the power output constant at a higher orlower value, e.g. for dimming the lamp.

The voltage of the power source may be raised from the above-assumedvalue of 110 V to 220 V or it may be lowered to a voltage merelyslightly above the rated value of the load, without damage and with avery small variation of the power output. Thus the circuit may beconnected to any common distribution network and it is not evennecessary to take notice of its voltage.

The types and values of the components mentioned above are given onlyfor illustrative purposes and do not involve any restriction of thescope of the invention. The following claims cover modifications invarious respects.

I claim:
 1. Circuitry for maintaining constant the effect drawn by aload from an A.C. power source essentially independent of its voltage,comprising:an operational amplifier having first and second inputs; apower transistor in series with the load from the A.C. power source; afeeding line carrying a feeding voltage of a sinusoidal, wave form;means for applying a fraction of the voltage of the A.C. power source tothe first input of the operational amplifier; means for applying areference voltage to the second input of the operational amplifier sothat the reference voltage is raised and lowered when the feedingvoltage is raised and lowered; and wherein the operational amplifier isconnected to the power transistor to control the power transistor, sothat the transistor is made leading when and as long as theinstantaneous value of said fraction of the feeding voltage is lowerthan a limiting value determined by said reference voltage; and so thatthe load is passed by periodic pulses of a curved form corresponding tothat of the first and last parts of a sine half-wave next to the zerovalue and of a length which is greater or smaller depending upon whethersaid feeding voltage is lower or higher, respectively; and so that theenergy contents of the pulses are maintained essentially constant over awide range of feeding voltage variations.
 2. Circuitry as recited inclaim 1 wherein the operational amplifier and power transistor areoperatively interconnected so that the length of the pulses is shortenedat a lower rate than in inverted proportion to the rise of the feedingvoltage, and vice-versa.
 3. Circuitry as recited in claim 1 wherein saidmeans for applying a reference voltage applies a reference voltage thatis composed of a first component maintained at a constant value by aZener diode, and a second component consisting of a smoothed fraction ofthe feeding voltage.
 4. Circuitry as recited in claim 1 wherein saidfirst input is an inverting input, and wherein said means for applying avoltage thereto comprises means for supplying an unsmoothed voltageforming a fixed fraction of the feeding voltage; and wherein said secondinput of said operational amplifier comprises a non-inverting input, andwherein said means for applying a voltage to said second input comprisesmeans for supplying a constant voltage superimposed by a smoothingfraction of the feeding voltage.